Magnetron oscillator of the resonant cavity type



April 13, 194s.

c. G. SMITH 2,439,401

MAGNETRON OSCILLATOR OF THE RESONANT CAVI'fY TYPE Filed sept. 1o, 1942 2 sheets-sheet 1 Apl 13, 1948.

IAGNETRON OSCILLATOR OF THE RESONANT CAVITY TYPE c. G. Smm 2,439,401

Filed Sept. 10, 1942 2 Sheets-Sheet 2 ,WVM/roe. CHAR/ Es 6I 15N/ TM 1 Patented 'Api'. 13, 1948 MAGNETRON osomLA'rn or 'ma nEsoNAN'r cAvrry 'rrrE l Charles G. Smith, Medford, Mass., assignor to Raytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application September 10, 1942, Serial No. 457,809

This invention relates to electrical space dis-.- charge devices which are adapted to produce pulses of ultra high .frequency oscillations, in which devices the oscillations are derived from the movement of electronsin curvilinear paths under the influence of la magnetic field, the anode forming one or more closed oscillatory tank circuits therein, the-frequency generated being principally determined by the geometry of the anode. q

An object of the present inventionis the provision `of an `improved electrical space discharge device of- `the type hereinabove described andof a method ofv operating same.

In certain pulsing applications, electrical space discharge devices of the aforementioned type produce oscillations 'in pulses involving the use .of -considerable power. An intense magnetic field is required to produce such oscillations. vHeretofore comparatively large magnets have been employedto obtain such intense magnetic field.

In accordance with my invention I am able to produce the required magnetic eld without the use of magnets. I accomplish this by means of surges of current of high value in a conductor. This conductor is preferably a straight conductor and may consist of the cathode-heating filament or of the cathode of the electrical space discharge device.

. In such pulsing applications, each of the pulses is of comparatively short duration with a comparatively long time interval between successive pulses. It is only during such pulses that the intense magnetic field for producing oscillations is required. Therefore, the surges of current for producing the magnetic field vpreferably are of the saine or approximately the same duration as said pulses, and are synchronized therewith. The amount of current supplied during such surges to establish the desired intense magnetic field may be such that despite the shortness of such surges enough energy is supplied to the cathode, or other conductor, to raise its temperature considerably. During the interval between successive surges this temperature falls. Due to the comparative length of such intervals it becomes feasible to maintainthe average temperature within practical limits. This can be accomplished by proper selection of the various constants of the system.

Another object of the present invention is therefore the provision of novel means for establishing the intense magnetic field required for producing the desired oscillations in a system of the type indicated.

11 Claims. (Cl. 250-'36) A further lobject of thepresent inveiiitionisv the provisionv of means for establishing :a magnetic field of novel coniigurationthereby en-V A ablingv the construction of a novel arrangement of an electrical space `discharge device of the type hereinabove indicated which has certain .adj vantages.

other and further' objects of the presen-.t ini vention will become apparent and the foregoing will be best understood `from the following de# scription of exemplications thereof, reference Abeing'had to the drawing in which f Fig. 1 is a perspective View of a longitudinal.

section ofan 'electricallspace discharge device embodying my invention, and a schematic' diagram of an electrical system in which said device is employed; and A lFig. 2 is a similar `view of a modified'. form .of

.such device. embodyingy my invention, and a, schematic diagram of an electrical system. ernployed therewith.

' Referring now to Fig. 1, the electrical space discharge device I there illustrated is comprised of an envelope 2 which may be made of a' block of .conductive material, lsuch as copper. "This,

envelope also serves as the anode of the device. Anode 2 is provided with a longitudinal central bore 3 within which is supported substantially at the center thereof a cathode 4. Aswill be explained .in detail hereinafter the cathode 4 is adapted to conduct pulses of current of high peak volume for brief periods of time. To prevent the cathode from becoming over-heated it .is Adesirable that the cathode dissipate energy as rapidly as possible during the periods between pulses in order that the temperature of the cathode may be confined within comparatively It is therefore desirable that the cathode utilized in this device .have the maximum surface with the minimum volume.

sleeve over a rod of quartz, Alundum, or thelike. The cathode Il` is maintained under tension and supported by a pair of conducting springs 5 arranged at opposite ends thereof, said springs in turn being connected with a pair of lead-in conductors 6 sealed through glass seals 1 mounted' For this purpose the cathode 4' may be made in the form of a ribbon or may consist of a thin metallic 3 slots 8 extending in a plane substantially normal to the longitudinal axis of the cathode. Adjacent pairs of such slots define between them annular segments 9. .Each pair of segments 9 separated by a slot 8 forms a tank circuit including inductance and capacity. When the aforedescribed device is energized in the presence of a suitable magnetic field, oscillations are set up Whose frequency is determined primarily by the dimensions and configurations hereinbefore described. The oscillations produced in these tank circuits reinforce each other. These oscillations may be led out of the devicev by means of a coupling loop I projecting into one of the slots l' 8. Loop II) may be connected with a lead-in conductor II arranged concentrically within a pipe I2 which pipe may be inserted within an opening in envelope 2, the pipe I2 being secured in said opening by a hermetically sealed thread. Conductor II may be sealed in a glass seal I3 mounted at the outer end of pipe I2. An additional conducting pipe I4 may be fastened to the pipe4 I2 outside of the seal so as to form with the lead-in conductor I I a concentric line through which the high frequency oscillations generated in the device may be conducted away andv utilized.

As has been stated before the electrical space discharge device here described is adapted to be used in pulsing applications, and the pulses `may be relatively of short duration, such as, for example, of the order oi microseconds. In producing these ultra high frequency oscillations an in tense magnetic eld is required during each pulse, but this eld is only required for short periods of time, In accordance with my invention the required magnetic eld is produced by large currents passing through the cathode 4, and since the magnetic iield is only required for short peincrease in emission is obtained without any substantial deleterious effects. I believe that this avoidance of substantial deleterious effects is due to the comparatively short duration of such surges, to the skin eifect of the high frequency of such surges and to other factors. This use of surges of current to'obtain larger emission is more fully described, and is claimed, in the copending application of Laurence K. Marshall for an improvement in the method of operating electrical space discharge devices, Serial No. 457,803, filed September 10, 1942.

The cathodeA may be heated to, or below, its rated operating Itemperature by any suitable means such as, for example, current derived from a battery I5. At this rated temperature the cathode may be incapable of delivering the emission required for producing the peak pulses. The surges of current for producing the desired magnetic field may be superimposed on the current supplied by battery I5 to obtain the required increased emissivity. These surges of current may ybe derived from the discharge of a fast condenser I6. By a fast condenser I mean a condenser having little distributed inductance. Condenser I6 may be charged from any suitable source of direct current supply I1 such as, for example, a directcurrent generator, rectifier, battery, or the like, and a current limiting resistance 33 may be interposed in series with thesource of charging current VSince condenser I6 isf'adapted to be v discharged ay great many times per second I prefer to utilize a vacuum tube relay I8 for controlling the discharge thereof. Since the ordinary vacuum tube relay while capable of handling high voltages may be incapable of handling the required amount of current necessary to produce the intense magnetic field and peak emission from the cathode 4, I prefer to have condenser I6 :discharge into the primary I9 of a closely coupled air core step-down transformer 20 having a. secondary 2I comprised of a few turns of wire. The circuit associated with the primary may be a high-voltage, low-current circuit, while the secondary will supply the required high current at a lower voltage. Close coupling between the primary and secondary is desired in order that the impedance or leakage inductance may be confined to a minimum. In order to obtain the desired close coupling the secondary 2I may consist of one or more turns of a conducting ribbon closely wound around the primary I9. The secondary 2| is connected to cathode 4 in series with battery I5. Since the usual battery has a large internal capacitance, it will oier little impedance to the pulses of vcurrent generated in the secondary 2l. It is preferred'that practically all the energy delivered by the discharge of condenser I 6 be supplied to the cathode 4 and that the impedance of the discharge circuit of condenser I6, outside of the impedance of the cathode 4, be limited to a minimum.

Electrical space discharge device is designed to produce oscillations in pulses. Pulsing potentials may besupplied between the anode 2 and the cathode 4 from any suitable source, such as a pulsing device 22. The surges of current to be supplied to the cathode 4 for establishing the magnetic fieldand for producing increased emission are preferably synchronized with these pulsing potentials. For this purpose vacuum tube relay I8 which controls the discharge of condenser I6 may be connected to the pulsing device 22. A phase control device 23 may be interposed between the pulsing device 22 and the electrical space discharge device I. This phase control device 23 may -be utilized to properly time the application of pulsing potentials in relation to surges of current derived from condenser I6.

In the illustrated pulsing system each peak pulse may have a duration of the order of a. microsecond, there being approximately 1000 such pulses per second, each pulse consisting of oscillations of high frequency, such as, for example, from about 300 to 1500 megacycles. The condenser discharge circuit may therefore have a time constant of the order of a microsecond. The time intervening between surges of energy supplied to the cathode by condenser I6 will be of the order of 1000 microseconds and the duration of each of such surges of energy relative to the time between such surges will be approximately of the order of 1 to 1000. It is of course apparent that these constants may be varied within considerably wide limitsv depending upon the particular conditions to be met. For example. the ratio vbetween the duration of said surges and the time between such surges maybe as low as 1 to 100 and practically may be as high as one to several thousands. Of course it is to be understood that a steady current may be used to produce the desired magnetic field instead of surge currents.

The discharge of condenser |6 is adapted to send a large pulse of current through the cathode 4. The maximum current derived from the discharge of condenser I6 is approximately a mlcrofarad, and preferably larger; and E may be 5000 volts, hence I r...x-5000 E?,

Under such conditions a field of from 2000 to 5000 gausses is readily attained. It will also be apparent that the emission obtained under these conditions from any suitable iilament will be high and therefore that tube I will provide a large output.

As will be readily understood by those versed in this art an electron emitted from the cathode 4 will be drawn towards the inner surface `of one of the segments 9. However, due to the concentric magnetic eld established by the current flowing through cathode 4, this electron will be deected in a direction parallel to the cathode towards an adjacent segment 9. For a -particular field strength, in the present instance deter- 5000 amperes mined primarily by the particular cathode current, and a deiinite voltage between the anode and the cathode, oscillations are generated in a manner similar to that occurring in a multiple plate magnetron. The `output power can e taken off by the coupling loop I0 and delive ,ed to the concentric line wave guide. l

The magnetic eld produced in this manner rises to a maximum and decreases to zero. Hence, for a single discharge from condenser I6 there may be two points at which the value of the magnetic eld is suitable for maximum generation of oscillation. Howevergthe voltage applied to the oscillating circuit through the pulsing device 22 may be regulated so that the maximum generation of oscillations is obtained at the time when said maximum field is established. It will be seen therefore that either one pulse of oscillation for each discharge will be produced, or a double pulse for each discharge will be produced.

In Fig. 2 there is illustrated a modified form of electrical space discharge device and of a system used in connection therewith. In the arrangement shown in* Fig. l, the anode 2 of electrical space discharge device I forms a plurality of tank circuits, each of the tank circuits defining separate oscillating regions. In the electrical space discharge device 26 of Fig. 2 the anode 21 forms but two tank circuits and two oscillating regions, but these oscillating regions are relatively much longer and enable the development of considerable power. By utilizing such an arrangement the energy of the oscillation in electrical space discharge device 26 may be easily transferred to the output coupling loop l0. Additional advantages are also obtained from this arrangement as will be seen from the following description thereof.

The anode 21 of electrical space discharge device 26 illustrated in Fig. 2 is provided with two parallel helical slots 28 and 29 in place of the separate radial slots 8 of the device of Fig. 1. These helical slots 28 and 29 are formed like a double screw thread, the two slots being adjacent each other and spiraling along the inside of the anode 21.` These slots define between them a helical segment and another helical segment 3|. The segments 30 and 3| might be considered as the walls of an elongated trough which has been twisted in the form of a helix. Two separate 'oscillating regions are thereby formed: one separate oscillating region consisting of the segment 30 and the segment 3| `and the slot 28 therebetween, and the other oscillating region consistingv of the segment 3| and the segment 30' and the slot 29 therebetween. These two oscillating regions are each relatively long and serve to reinforce one another.

In the arrangement of Fig.,2 in addition to the magnetic field produced by the energy lderived from the discharge of condenser I6, an additional ,i means is provided for producing a second magnetic field extendingparallel to the cathode. This second magnetic field may be generated by current from any suitable source of current supply 30 which is passed through a coil 3| arranged about the tube 2, the strength of this iield being controlled by any suitable means, such as a rheostat 32 arranged in series with the source of current 30'. The first mentioned field deflects electrons emitted from the cathode in a direction longitudinally of the cathode. The second field imparts an additional deiiection of the electron in a direction concentrically about said cathode. The combination of the two iields produces a lspiral magnetic field which upon proper selection of the parameters will be 'approximately parallel to the spiral 0f Segments 29. In this arrangement a slight change in the strength of one of the i'lelds relative to the other will produce a considerable change in the output.

Thus, a simple means for modulatingthe output is provided. Any suitable modulation device 3| may be arranged in series with the source of current 30' to vary the current passing through coil 30. Other details of the system associated with tube 26Vmay be similar to those described in connection with Fig. l.

While I have described specic embodiments of my invention, it will be apparent that numerous modifications may be made therein without departing from my invention. For example,

tube may have its anode made of a plurality ment of electrons in curvilinear paths under thev iniiuence of a magnetic iield, said device comprlslng an elongated cathode, a plurality of helical anode elements arranged about said cathode substantially concentric therewith, and means for establishing a magnetic field having its lines of force enclosing said cathode and having components of said field lying at right angles to the longitudinal axis of said cathode.

2. An electronic device adapted to cause movement of electrons in curvilinear paths under the inuence of a magnetic held, said device' comprising an elongated cathode, a pair or elongated helical anode elements arranged in the iorm oi a double screw thread enclosing said cathode. and substantially concentric therewith, and means for establishing a magnetic field having its lines of force enclosing said cathode and having components of said iield lying at right angles to the longitudinal axis oi said cathode.

3. An electronic device adapted to cause movement of electrons in curvilinear paths under the inuence of a magnetic field, said device comprising an elongated cathode, a plurality or helical anode elements arranged about said cathode substantially concentric therewith, and means for establishing a magnetic ileld having its lines of force enclosing said cathode. and having certain components of said eld lying at right angles to the longitudinal axis of said cathode, and other components lying in a plane parallel to the longitudinal axis of said cathode.

4. An electronic device adapted to cause movement oi electrons in curvilinear paths under the influence of a magnetic ileld, said device coming path oi a'tank circuit arranged adjacent said cathode, electrical energy storage means, means for charging said electrical energy storage means, and a circuit for discharging said electrical energy storage means through said cathode to establish a magnetic eld having its lines oi force circling about said cathode. and to raise the temperature of said cathode.

8. An electronic device adapted to cause movement of electrons in curvilinear paths under the iniluence of a magnetic held, said device comprising an elongated cathode. an anode element, and means for establishing a periodically uctuating magnetic field, said means including means for periodically supplying surges of current to said cathode. ,t t

9. An electronic device adapted to generate oscillations derived from the movement oi' electrons in curvilinear pathsA under the influence of a.LHV magnetic field, said device comprising an elongated cathode, an anode element, and means for establishing a magnetic ileld to generate said oscillations, said means including means for supy plying sufdcient current to said cathode to D10.

prising an elongated cathode, a plurality oi heliv cal anode elements arranged about said cathode substantially concentric therewith. means for establishing a magnetic field having spiral lines oi force enclosing said cathode, said lines of force being substantially normal to the anode elements.

5. An electronic device adapted to cause movement of electrons in curvilinear paths under the influence of a magnetic field. said device having a plurality of elements therein including a cathode and a plurality of anode elements arranged in a row adjacent said cathode, said cathode being in the form of an elongated conductor, electrical energy storage means, means for storing energy in said electrical storage means, and a circuit for discharging said electrical storage energy means through said elongated conductor to establish a magnetic field having its lines of force enclosing said elongated conductor, and having components of said eld lying at right angles to Y the longitudinal vaxis of said conductor.

6. An electronic device adapted to cause movement of electrons in curvilinear paths under the iniiuence of a. magnetic eld. said device comprising an elongated cathode,v a plurality of anode elements arranged in a row extending lengthwise of and adjacent to said cathode, and means for establishing a magnetic ileld having its lines oi.' force enclosing said cathode and having components of .said field lying at right angles to the longitudinal axis of said cathode, said means including an electrical energy storage means, means for supplying energy to said electrical energy storage means, and a circuit for discharging-said electrical energy storage means through said cathode.

'7. An electronic ydevice adapted to cause movement of electrons in curvilinear paths under the influence of a magnetic field. said device comprising an elongated cathode, an anode element formduce a magnetic field sufficiently intense to generate oscillations.

l0. An electronic device adaptedA to generate oscillations derived from the movement of electrons in curvilinear paths under the inuence of a magnetic iield, said device comprising a plurality of electrodes including a cathode element and an anode element adjacent thereto,v said cathode being an elongated conductor, and means for establishing a magnetic ileld for the generation of oscillations, said means including means for supplying suiiicient current to said elongated conductor to produce `a magnetic field sufdciently intense to generate said oscillations.

` l1. An electronic device adapted to cause movement of electrons in curvilinear paths under the influence of a magnetic field, said ydevice comprising a plurality o! electrodes including a cathode element and an anode element adjacent thereto, said cathode being an elongated conductor, and means for establishing a periodically iluctuating magnetic field for the generation of oscillations, said means including means for periodically supplying surges of current tosaid elongated conductor.

. CHARLES G. SMITH.

REFERENCES crrED The following references are of record in the ille of this patent:

Certificate of Correction Patent No. 2,439,401. April 13, 1.948.

CHARLES G. SMITH l It is hereby certified that error appears in the printed specification ofthe above numbered patent requiring correction as follows: Column 8, line 1, claim 7, forI the word pathpread part; and that the said Letters Patent should be read with this ciflrection therein that the same may conform to the record of the case in the Patent Signed and sealed this 8th day of June, A.` D. 1948.

[smh] THOMAS F. MURPHY,

" Anatafnt'onunaonerofPaenta. 

